Use of Composites for Aircraft Primary Structural Components

In today’s commercial aviation world, airlines have for a long time understood the importance of flying an aircraft as economically as possible. Advances in technology have made this possible in a number of ways, one of which is the introduction of composite material use wherever feasible. Composite materials typically offer a weight saving of between 20 and 25% when used in place of historically manufactured components made predominantly from alloyed metals. The heavier the aircraft, the more fuel it burns for a given mission, making weight reduction top priority for aircraft designers. The non-corrosive benefits, high-energy absorption and resistance to fatigue offered by composites are another attractive feature, but despite this and the favourable strength to weight ratio, they aren’t the miracle solution for all aircraft structures. Composites can be difficult to inspect for flaws, some absorb moisture, which compromises their structural integrity. The fabrication process is often complex and labour intensive, requiring the use of specialist equipment and expertise thus making utilization cost prohibitive.

In its most primitive form a composite material consists of two dissimilar materials, one being the matrix and the other the reinforcement. These materials are selected so as to mechanically compliment one another whilst neutralising their deficiencies. The reinforcement may have high strength in tension, but little resistance to bending and compressive forces, the matrix may on the other hand have high resistance to bending and compressive forces, so when used together, produce a composite with both high tensile and compressive strengths and also a high resistance to bending. The matrix binds together the reinforcement, which is usually in the form of rods, strands, fibres or particles and is much stronger and stiffer than the matrix, this results in the reinforcement being held in an orderly pattern and because the reinforcements are usually discontinuous, the matrix also helps to transfer load and protects the reinforcement from the environment.

There are three types of composite materials, Polymer Matrix Composites (PMC), Metal Matrix Composites (MMC) and Ceramic Matrix Composites (CMC).

Most commonly used are PMC’s where the matrix is often polyester, vinyl ester or epoxy resin. Polymers are not the strongest of materials but provide excellent ability to be moulded when in the thermosetting form. The reinforcing materials widely used for PMC’s are glass; carbon, aramid and boron, all of which are in fibre form available at different strengths denoted by a graded “modulus”.

MMC’s are used chiefly to strengthen low-density metals such as copper and alloys of aluminium, titanium or magnesium. They possess higher wear and temperature resistance, but are complex and expensive to produce, a factor that often limits their use. Typical materials used to reinforce an MMC are Boron/Tungsten, Titanium, Alumina, Graphite and Silicon Carbide.

CMC’s are highly advanced composite materials, used in areas where high strength is required at high temperatures, The ceramic matrix is reinforced with Silicon Carbide, Boron Nitride or alternative ceramic fibres, their high production cost limits their use to essential applications.

Composites have been used in aeroplanes since the 1950’s with critical applications introduced in the early 1980’s. It is not uncommon to find composites extensively used in both secondary and tertiary aircraft structures, such as internal cabin panels, cowlings, access panels and fairings in non-pressurised areas. But as confidence over the years has grown, we find an increasing use of composite materials in primary aircraft structure. A primary structure is defined as any structural part of the aircraft where no backup exists and affects the safety of the aircraft if catastrophic failure occurs. Beginning with applications in moveable flying control surfaces and the vertical stabiliser, and progressing to use in an increasing number of primary structural components including the keel beam and rear pressure bulkhead of the A340-600, the civil aviation industry has adopted an evolutionary approach to the uptake of composites. Figure 1 illustrates the growth of composite use in civil transport aircraft since 1980.

Figure 1 The growth of composite use in civil transport aircraft since 1980 1

The Boeing 777 made its maiden flight over a decade ago on the 12th of June 1994 and in terms of primary structure included a composite vertical and horizontal stabiliser, as well as all flight control surfaces being of composite construction. The following quote illustrates Boeings increased use of composite materials on the 777 as compared